Passing a dynamic parameter throws RuntimeBinderException when calling Method from Inherited interface

Ran into an interesting runtime issue after some refactoring and have pinned in down to the following situation. When passing a property from a dynamic object to a method on an Interface that has been inherited from a parent interface the runtime binder cannot find the method. Here is a test to demonstrate both failure and success (when calling method directly on the parent interface type) ``` using System.Dynamic; using Microsoft.CSharp.RuntimeBinder; using Microsoft.VisualStudio.TestTools.UnitTesting; namespace Test.Utility { public interface IEcho { string EchoString(string input); } public interface IInheritEcho : IEcho { } public class EchoClass : IInheritEcho { public string EchoString(string input) { return input; } } [TestClass] public class RuntimeBinderTest { [TestMethod] public void RuntimeBinder_should_work_when_dynamic_parameters_are_passed_to_method_from_inherited_interface() { //Arrange dynamic dynObject = new ExpandoObject(); dynObject.Foo = "Bar"; IInheritEcho echomore = new EchoClass(); string echo = null; string exceptionMessage = null; //Act try { echo = echomore.EchoString(dynObject.Foo); } catch (RuntimeBinderException e) { exceptionMessage = e.Message; } //Assert Assert.AreEqual(echo, dynObject.Foo, false, exceptionMessage); } [TestMethod] public void RuntimeBinder_should_work_when_dynamic_parameters_are_passed_to_method_from_noninherited_interface() { //Arrange dynamic dynObject = new ExpandoObject(); dynObject.Foo = "Bar"; IEcho echomore = new EchoClass(); string echo = null; string exceptionMessage = null; //Act try { echo = echomore.EchoString(dynObject.Foo); } catch (RuntimeBinderException e) { exceptionMessage = e.Message; } //Assert Assert.AreEqual(echo, dynObject.Foo, false, exceptionMessage); } } } ``` Test #1 Fails: Assert.AreEqual failed. Expected:<(null)>. Actual:. 'Test.Utility.IInheritEcho' does not contain a definition for 'EchoString' Test #2 Succeeds. My question is whether my assumption that the 1st test should pass is correct or is there a fundamental reason in the framework that it does not? I know I can fix the issue by casting the parameters when I pass them in or assigning them to variables before passing them in. I'm more just curious as to the reason the inherited interface is causing the RuntimeBinder to fail...